Perovskite material, preparation method and use in catalytic membrane reactor

ABSTRACT

The invention concerns a mixed electronic and O 2−  anion conductive perovskite material, of formula (I): A (a)   (1-x-u) A′ (a−1)   x A″ (a″)   u B (b)   (1-s-y-v) B (b+1)   s B′ (b+β)   y B″ (b″)   v O 3-d , wherein: a, a−1, a″, b, b+1, b+β et b″ are integers representing respective valences of the atoms A, A′, A″, B, B′, B″; a, a″, b, b″, β, x, y, s, u, v et δ such that the electrical neutrality of the crystal lattice is preserved; A represents an atom selected among scandium, yttrium or in the families of lanthanides, actinides or alkaline-earth metals; A″ represents an atom selected among Al, Ga, In, or Tl; B, B′, B″ represents an atom selected among the transition metals, Al, In, Ga, Ge, Sb, Bi, Sn or Pb. The invention also concerns the method for preparing said material and its use as mixed conductive material of a catalytic membrane reactor, for use in synthesizing synthetic gas by oxidation of methane or natural gas.

The subject of the present invention is a mixed (electronic/O²⁻anion)conductive material of perovskite structure, its method of preparationand its use in a catalytic membrane reactor for carrying out theoperation of reforming methane or natural gas into syngas (H₂/COmixture).

Catalytic membrane reactors, hereafter called CMRs) formed from suchceramic materials allow the separation of oxygen from air, the diffusionof this oxygen in ionic form through the ceramic material and thechemical reaction of the latter with natural gas (mainly methane) oncatalytic sites (Ni or noble metal particles) deposited on the membrane.The conversion of syngas into liquid fuel by the GTL (Gas to liquid)process requires an H₂/CO molar ratio of 2. This ratio of 2 can beobtained directly by a process involving a CMR.

The perovskite is a mineral of formula CaTiO₃ having a specific crystalstructure that can be identified by XRD (X-ray diffraction). The unitcell of this compound is a cube whose corners are occupied by the Ca²⁺cations, the center of the cube by Ti⁴⁺ cation and the center of thefaces by the O²⁻ oxygen anions.

Oxides of the perovskites family are represented by the general formulaABO₃ in which A and B are metal cations, the sum of the charges of whichis equal to +b. In principle, A is an element of the lanthanide groupand B is a transition metal. By extension, any compound of formula ABO₃,in which A and B may be the abovementioned chemical elements or mixturesof these elements with other cations, and having the crystal structuredescribed above, is called a perovskite.

The partial substitution of the elements A and B with elements A′ and B′in order to form a perovskite compound of the A_(1-x)A′_(x)B_(1-y)B′yO₃type entails many modifications within the material that it may prove tobe particularly advantageous for the intended application.

U.S. Pat. No. 5,648,304 and U.S. Pat. No. 5,911,860 disclose mixedconductive materials of perovskite structure. However, these materialsdo not have a formulation and a method of synthesis that are suitablefor optimum performance in a CMR application.

The Applicant therefore aims to develop a novel material displayinggreater ionic conductivity than those of the prior art while stillpreserving stability over time.

Therefore, according to a first aspect, the subject of the invention isa mixed electronic/O²⁻-anion conductive material of perovskite crystalstructure, characterized in that it consists essentially of a compoundof formula (I):_(u)A^((a)) _((1-x-x)A″^((a″)) ₎A′^((a−1))B^((b)) _(u(1-s-y-v))B^((b+1))_(s)B′^((b+β)) _(y)B″^((b″)) _(v)O_(3-δ),  (I),in which formula (I):

a, a−1, a″, b, b+1, b+β and b″ are integers representing the respectivevalences of the atoms A, A′, A″, B, B′ and B″; and a, a″, b, b″, β, x,y, s, u, v and δ are such that the electrical neutrality of the crystallattice is preserved;a>1;a″, b and b″ are greater than zero;−2≦β≦2;a+b=6;0<s<x;0<x≦0.5;0≦u≦0.5;(x+u)≦0.5;0≦y≦0.9;0≦v≦0.9;0≦(y+v+s)≦0.9;[u(a″−a)+v(b″−b)−x+s+βy+2δ]=0;and δ_(min)<δ, <δ_(max) withδ_(min) =[u(a−a″)+v(b−b″)−βy]/2 andδ_(max) =[u(a−a″)+v(b−b″)−βy+x]/2;and in which formula (I):

A represents an atom chosen from scandium, yttrium or from the familiesof lanthanides, actinides or alkaline-earth metals;

A′, which differs from A, represents an atom chosen from scandium,yttrium or from the families of lanthanides, actinides or alkaline-earthmetals;

A″, which is different from A and A′, represents an atom chosen fromaluminum (Al), gallium (Ga), indium (In) and thallium (Tl) or from thefamilies of alkaline-earth metals;

B represents an atom chosen from the transition metals that can exist inseveral possible valences;

B′, which differs from B, represents an atom chosen from transitionmetals, aluminum (Al), indium (In), gallium (Ga), germanium (Ge),antimony (Sb), bismuth (Bi), tin (Sn), lead (Pb) and titanium (Ti); and

B″, which differs from B and B′, represents an atom chosen fromtransition metals, metals of the alkaline-earth family, aluminum (Al),indium (In), gallium (Ga), germanium (Ge), antimony (Sb), bismuth (Bi),tin (Sn) and lead (Pb) or titanium (Ti).

The expression “family of alkaline-earth metals” is understood to mean,in the case of A, A′ or B″, an atom essentially chosen from magnesium(Mg), calcium (Ca), strontium (Sr) and barium (Ba).

The expression “family of lanthanides” is understood to mean, in thecase of A, an atom essentially chosen from lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (EU),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium, erbium (Er),thulium (Tm), ytterbium (Yb) and lutetium (Lu).

The expression “transition metals that can exist in several possiblevariances” is understood to mean, in the case of B, metals possessing atleast two possible adjacent oxidation states, and more particularly anatom chosen from titanium (Ti), vanadium (V), chromium (Cr), manganese(Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zirconium (Zr),molybdenum (Mo), ruthenium (Ru), rhodium (Rh), tantalum (Ta), tungsten(W), rhenium (Re), osmium (Os), iridium (Ir) and platinum (Pt).

The term “transition metal” is understood to mean, in the case of B′ orB″, an atom essentially chosen from titanium (Ti), vanadium (V),chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni),copper (Cu) zinc (Zn), zirconium (Zr), niobium (Nb), molybdenum (Mo),ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), hafnium (Hf),tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir),platinum (Pt) and gold (Au).

According to a first particular aspect, the object of the invention is amaterial as defined above, for which, in formula (I), δ is equal to anoptimum value δ_(opt) that allows it to ensure an optimum ionicconductivity for sufficient stability under operating temperature andpressure conditions as a mixed ionic/electronic conductor.

As will be explained here, the diffusion of oxygen into the materialwhich is the subject the present invention is facilitated by thepresence of oxygen vacancies in the crystal lattice. Now, it has beenfound that the simple choice of the chemical composition in terms of theelements A, A′, A″, B, B′ and B″ does not fix the number of oxygenvacancies and that, consequently, this is not a sufficient condition toensure both good ionic conductivity and good stability under the normalconditions of use, especially an operating temperature between about600° C. and 1000° C.

According to a second particular aspect, the subject of the invention isa material as defined above, for which, in formula (I), a and b areequal to 3.

According to a third particular aspect, the subject of the invention isa material as defined above, in which, in formula (I), u is equal tozero.

According to a fourth particular aspect, the subject of the invention isa material as defined above, in which, in formula (I), u is differentfrom zero.

According to a fifth particular aspect, the subject of the invention isa material as defined above, for which, in formula (I), the sum (y+v) isequal to zero.

According to a sixth particular aspect, the subject of the invention isa material as defined above, for which, in formula (I), the sum (y+v) isdifferent from zero.

According a seventh particular aspect, the subject of the invention is amaterial as defined above, for which, in formula (I), A is chosen fromLa, Ce, Y, Gd, Mg, Ca, Sr or Ba and more particularly a material offormula (Ia):La^((III)) _((1-x-u))A′^((II)) _(x)A″^((a″)) _(u)B^((III))_((1-s-y-v))B^((IV)) _(s)B′^((3+β)) _(y)B″^((b″)) _(v)O_(3-δ)  (Ia),corresponding to formula (I) in which a and b are equal to 3 and Arepresents a lanthanum atom.

According to an eighth particular aspect, the subject of the inventionis a material as defined above, for which, in formula (I), A′ is chosenfrom La, Ce, Y, Gd, Mg, Ca, Sr or Ba and more particularly a material offormula (Ib):A^((III)) _((1-x-u))Sr^((II)) _(x)A″^((a″)) _(u)B^((III))_((1-s-y-v))B^((IVD) _(s)B′^((3+β)) _(y)B″^((b″)) _(v)O_(3-δ)  (Ib),corresponding to formula (I) in which a and b are equal to 3 and A′represents a strontium atom.

According to a ninth particular aspect, the subject of the invention isa material as defined above, for which, in formula (I), B is chosen fromFe, Cr, Mn, Co, Ni and Ti and more particularly the subject is amaterial of formula (Ic):A^((III)) _((1-x-u))A′^((II)) _(x)A″^((a″)) _(u)Fe^((III))_((1-s-y-v))Fe^((IV)) _(s)B′^((3+β)) _(y)B″^((b″)) _(v)O_(3-δ)  (Ic),corresponding to formula (I) in which b=3 and B represents an iron atom.

According to a tenth particular aspect, the subject of the invention isa material as defined above, for which, in formula (I), B′ is chosenfrom Co, Ni, Ti, Mn, Cr, Mo, Zr, V and Ga.

According to an eleventh particular aspect, the subject of the inventionis a material as defined above, for which, in formula (I), B″ is chosenfrom Ti of Ga and more particularly the subject is a material of formula(Id): La^((III)) _((1-x))Sr^((II)) _(x)Fe^((III)) _((1-s-v))Fe^((IV))_(s)B″^((b″)) _(v)O_(3-δ) (Id),

Corresponding to formula (I), in which a=b=3, u=0, B represents an ironatom, A a lanthanum atom and A′ a strontium atom.

According to a twelfth particular aspect, the subject of the inventionis a material as defined above, for which, in formula (I), A″ is chosenfrom Ba, Ca, Al and Ga.

As examples of materials there are those for which formula (I) iseither:La^((III)) _((1-x-u))Sr^((II)) _(x)Al^((III)) _(u)Fe^((III))_((1-s-v))Fe^((IV)) _(s)Ti_(v)O_(3-δ),La^((III)) _((1-x-u))Sr^((II)) _(x)Al^((III)) _(u)Fe^((III))_((1-s-v))Fe^((IV)) _(s)Ga_(v)O_(3-δ),La^((III)) _((1-x))Sr^((II)) _(x)Fe^((III)) _((1-s-v))Fe^((IV))_(s)Ti_(v)O_(3-δ),La^((III)) _((1-x))Sr^((II)) _(x)Ti^((III)) _((1-s-v))Ti^((IV))_(s)Fe_(v)O_(3-δ),La^((III)) _((1-x))Sr^((II)) _(x)Fe^((III)) _(1-s-v))Fe^((IV))_(s)Ga_(v)O_(3-δ) orLa^((III)) _((1-x))Sr^((II)) _(x)Fe^((III)) _((1-s))Fe^((IV))_(s)O_(3-δ),and more particularly that of formula (Id) as defined above, in which xis equal to 0.4, B″ represents a trivalent gallium atom, v is equal to0.1 and δ=0.2−(s/2) and preferably that in which δ is preferably equalto δ_(opt)=0.180±0.018.

The subject of the invention is also a method of preparing a mixedelectronic/O²⁻ anion conductive material of perovskite crystalstructure, the electrical neutrality of the crystal lattice of which ispreserved, represented by the crude formula (I′):A_((1-x-u))A′_(x)A″_(u)B_((1-y-v))B′_(y)B″_(v)O_(3-δ),  (I′)in which formula (I′):

x, y, u, v and δ are such that the electrical neutrality of the crystallattice is preserved;0<x≦0.5;0≦u≦0.5;(x+u)≦0.5;0≦y≦0.9;0≦v≦0.9;0≦(y+v)≦0.9; and0<δand in which formula (I′):

A represents an atom chosen from scandium, yttrium or from the familiesof lanthanides, actinides or alkaline-earth metals;

A′, which differs from A, represents an atom chosen from scandium,yttrium or from the families of lanthanides, actinides or alkaline-earthmetals;

A″, which is different from A and A′, represents an atom chosen fromaluminum (Al), gallium (Ga), indium (In) and thallium (Tl);

B represents an atom chosen from the transition metals that can exist inseveral possible valences;

B′, which differs from B, represents an atom chosen from transitionmetals, aluminum (Al), indium (In), gallium (Ga), germanium (Ge),antimony (Sb), bismuth (Bi), tin (Sn) and lead (Pb); and

B″, which differs from B and B′, represents an atom chosen fromtransition metals, metals of the alkaline-earth family, aluminum (Al),indium (In), gallium (Ga), germanium (Ge), antimony (Sb), bismuth (Bi),tin (Sn) and lead (Pb);

characterized in that it comprises the following successive steps:

a step (a) of synthesizing a powder having an essentially perovskitecrystal phase from a blend of compounds consisting of at least onecarbonate and/or of an oxide and/or of a nitrate and/or of a sulfateand/or of a salt of each of the elements A, A′ and B and, if necessary,of a carbonate and/or of an oxide and/or of a nitrate and/or of asulfate and/or of a salt of A″, B′ and/or B″;

a step (b) of forming the powder blend obtained from step (a);

a step (c) of removing the binder from the formed material obtained fromstep (b); and

a step (d) of sintering the material obtained from step (c);

and characterized in that at least one of steps (a), (c) and (d) iscarried out while controlling the oxygen partial pressure (pO₂) of thegaseous atmosphere surrounding the reaction mixture.

In formula (I′) as defined above, A is more particularly chosen from La,Ce, Y, Gd, Mg, Ca, Sr and Ba and, in this case, the material prepared bythe method as defined above is preferably a material of formula of(I′a):La_((1-x-u))A′_(x)A″_(u)B_((1-y-v))B′_(y)B″_(v)O_(3-δ)  (I′a)corresponding to formula (I′) in which A represents a lanthanum atom.

In formula (I′) as defined above, A′ is more particularly chosen fromLa, Ce, Y, Gd, Mg, Ca, Sr, and Ba and, in this case, the materialprepared by the method as defined above is preferably a material offormula (I′b):A_((1-x-u))Sr_(x)A″_(u)B_((1-y-v))B′_(y)B″_(v)O_(3-δ)  (I′b),corresponding to formula (I′), in which a and b are equal to 3 and A′represents a strontium atom.

In formula (I′) as defined above, B is more particularly chosen from Fe,Cr, Mn, Co, Ni and Ti and, in this case, the material prepared by themethod as defined above is preferably a material of formula (I′c):A_((1-x-u))A′_(x)A″_(u)Fe_((1y-v))B′_(y)B″_(v)O_(3-δ)  (I′c),corresponding to formula (I′) in which b=3 and B represents an ironatom.

The method as defined above is preferably used to prepare a material offormula (I′d):La_((1-x))Sr_(x)Fe_((1-v))B″_(v)O_(3-δ)  (I′d),corresponding to formula (I′) in which a=b=3, u=0, y=0, B represents aniron atom, A a lanthanum atom, A′ a strontium atom and B″ is chosen fromTi and Ga. In general, before step (a) of the method defined above iscarried out, the high-purity precursor powders are washed beforehandand/or dried and/or heated to 600° in order to extract the volatilecompounds and the adsorbed water. They are then weighed and mixed in theappropriate proportions for obtaining the desired blend. The blend ofprecursors is then milled by attrition in the presence of a solvent, inorder to obtain a fine homogeneous blend. After drying, the resultantpowder is subject to step (a).

Step (a) generally consists of a calcination, which takes place in atemperature generally between 800° C. and 1500° C., preferably between900 and 1200° C., for 5 h to 15 h in air or in a controlled atmosphere.XRD analysis is then used to verify the state of reaction of thepowders. If necessary, the powder is milled further and then calcinedaccording to the same protocol until the precursors have completelyreacted and the desired perovskite phase has been obtained.

After step (a) of the method as defined above, the powder has apredominantly perovskite phase and possibly a small amount of secondaryphases (reactivity between some of the precursors, resulting insuboxides) varying between 0 and 10% by volume. The nature and thefraction of these phases may vary depending on the temperatures reached,on the homogeneity of the blend or the type of atmosphere used.

After the forming step (b), the powder formed may be milled in order tomatch the size, shape and specific surface area of the grains to theforming protocol used. The particle size of the powder is checked byparticle size analysis or by SEM or by any other specific apparatus.

The forming step (b) may consist of:

an extrusion operation, for example to form cellular structures orsheets or tubes;

a coextrusion operation, for example to form porous tubes or sheets or adense membrane;

a pressing operation, for example to form tubes or disks or cylinders orsheets; or

a strip casing operation, for example to form sheets that maysubsequently be cut up.

These methods in general require additions of organic compounds such asbinders and plasticizers that impart flow properties suitable for theprocess and favorable mechanical properties so that the object can behandled in the green state, that is to say before sintering.

The removal of the organic components requires a heat treatment stepprior to sintering. This step (c), called the binder removal step, iscarried out in an oven in air or in a controlled atmosphere, with asuitable thermal cycle, generally by pyrolysis with a slow heating rateup to a hold temperature of between 200 and 700° C., preferably between300° C. and 500° C. After this step, the relative density of themembrane must be at least 55% in order to facilitate densification ofthe object during sintering.

The sintering step (d) is carried out between 800 and 1500° C.,preferably between 1000° C. and 1400° C. for 2 to 3 hours in acontrolled (pO₂) atmosphere and on a support between which and thematerial there is little or no interaction. Supports made of aluminum(Al₂O₃) or magnesium (MgO), or a bed of coarse powder of the samematerial, will therefore be preferably used. After this step, themembranes must be densified to at least 94% so as to be impermeable toany type of molecular gas diffusion.

According to a first particular way of implementing the method asdefined above, the powder obtained at step (a) is formed by tape casting(step b). By introducing suitable organic compounds as binder (forexample a methacrylate resin or PVB), dispersants (for example aphosphoric ester) and plasticizer (for example dibutylphthalate) it ispossible to obtain a tape of controlled thickness (between 100 and 500μm). This tape may be cut into disks 30 mm in diameter. These disks maybe stacked and thermocompression-bonded at 65° C. under a pressure of 50MPa for 5 to 6 minutes so as to obtain greater thicknesses. Themembranes then undergo the binder removal step (step c) and are sintered(step d).

According to a second particular way of implementing the method asdefined above, step (c) is carried out while monitoring the oxygenpartial pressure (pO₂) of the gaseous atmosphere surrounding thematerial undergoing binder removal.

In a third particular way of implementing the method as defined above,step (d) is carried out in a gaseous atmosphere having a controlledoxygen partial pressure of between 10⁻⁷ Pa and 10⁵ Pa, preferable closeto 0.1 Pa, and in this case step (a) is preferably carried out in air.

According to another aspect, the subject of the invention is a materialof formula (I′), as defined above, and particularly a material offormula (I′a), (I′b), (I′c) or (I′d) in which δ depends on the oxygenpartial pressure in the gaseous atmospheres in which steps (a), (d) andoptionally (c) of the method as defined above take place.

Finally, the subject of the invention is the use of the material asdefined above as mixed conductive material (electronic and ionicconductor) of a catalytic membrane reactor designed to be used tosynthesize syngas by the oxidation of methane or natural gas.

FIG. 1 is a schematic representation of the anion and electron diffusionthrough the catalytic membrane reactor in operation.

The following description illustrates the invention without howeverlimiting it.

Preparation of a Material of Formula La_(0.6) Sr_(0.4) Fe_(0.9) Ga_(0.1)O_(3-δ)

Synthesis of the Material

A powder blend, preheated in order to remove any residual water orgaseous impurities, was prepared, said blend comprising:

44.18 g of La₂O₃ (from Ampere Industrie™: purity >99.99% by weight);

26.69 g of SrCO₃ (from Solvay Baris™: purity >99% by weight);

32.81 g of Fe₂O₃ (from Alfa Aesar™; purity >99% by weight);

4.28 g of Ga₂O₃ (from Sigma Aldrich™; purity >99% by weight).

The blend was milled in a polyethylene jar fitted with a rotating blademade of the same material together with spherical balls made ofyttria-stabilized zirconia (YSZ), an aqueous or organic solvent andoptionally a dispersant.

This attrition milling resulted in a homogeneous blend of powderparticles of smaller diameter and of relatively spherical form and witha monomodal particle size distribution. After this first millingoperation, the mean diameter of the particles was between 0.3 μm and 2μm. The contents of the jar were passed through a screen with a meshsize of 200 μm in order to separate the powder from the balls. Thisscreened powder was dried and stored before being calcined.

The powder blend obtained was calcined on an alumina refractory in afurnace. The partial oxygen pressure of the atmosphere was set bycirculating an appropriate gas or gas mixture in the furnace. The oxygenpartial pressure was monitored so as to remain within the [10⁻⁷ Pa to10⁵ Pa] range. The furnace was flushed with a gas mixture before thetemperature rise was started, in order to establish the desired partialoxygen pressure, this being monitored by an oxygen probe or achromatograph placed at the outlet of the furnace.

The gas mixture was composed of 0 to 100% oxygen, the balance beinganother type of gas, preferably argon or nitrogen or carbon dioxide. Thetemperature was then increased up to a hold temperature between 900° C.and 1200° C. and held there for 5 h to 15 h. The rate of temperaturerise was typically between 5° C./min and 15° C./min, while the rate offall was governed by the natural cooling of the furnace.

XRD analysis was then used to check the state of reaction of thepowders. Optionally, the powder was further milled and/or calcined usingthe same protocol until the reaction of the precursors was complete andthe desired perovskite phase obtained.

The perovskite powder obtained was formed by the conventional methodsused for ceramics. Such methods systematically rely on additions oforganic compounds that have to be extracted by pyrolysis (step c: binderremoval) before the actual sintering step and high temperature (step d).

The resulting ceramic part was introduced into the furnace, the oxygenpartial pressure of which was controlled as in the previous calcinationstep. The temperature was increased slowly, at about 0.1° C./min to 2°C./min until a first hold temperature of between 300° C. and 500° C. wasreached (the binder removal step c). The hold time varied between 0 and5 h depending on the conditions used and the volume of the part. Thisoperation was carried out either in a controlled atmosphere or anuncontrolled atmosphere. The oxygen content was between 10⁻⁷ Pa and 10⁵Pa, preferably not exceeding 0.1 Pa. Once the oxygen partial pressure ofthe enclosure had been established, the temperature was increased up tothe sintering temperature, generally between 1000° C. and 1400° C. witha hold lasting 1 to 3 hours, the oxygen partial pressure in the furnacebeing controlled. Upon return to room temperature, the relative densityof the parts was checked, and also the absence of cracks, in order toguarantee impermeability of the membrane.

The two main preparation steps (synthesis (step a) and sintering (stepd)) were carried out in air (pO₂=2·10⁴ Pa or in nitrogen (pO₂=0.1 Pa).The temperatures at which the flows were measured varied between 500 and1000° C. The oxidizing and reducing gases used in this example were airand argon, respectively. The measurements were carried out over severalhours of operation.

The oxygen contents in the argon downstream of the thermal chamber weremeasured using an oxygen probe and/or a gas chromatograph (GC).

Table 1 shows the influence of the synthesis protocol on a materialdescribed in the present invention.

FIG. 5 shows the stability of the oxygen permeation flux over more than100 h of operation for an air/argon mixture at 1000° C. and atmosphericpressure on both sides. TABLE 1 Oxygen flux Oxygen flux pO₂ at 500° C.at 1000° C. Protocol Synthesis (Pa) (Nm³/m²/h) (Nm³/m²/h) P1 Calcination2 · 10⁴ ≈0 0.17 Sintering 2 · 10⁴ P2 Calcination 2 · 10⁴ 0.10 0.51Sintering 0.1 P3 Calcination 0.1 ≈0 0.18 Sintering 2 · 10⁴ P4Calcination 0.1 1.5 then CMR Sintering 0.1 0.25 cracking (unstablesystem)Characterization by X-Ray Diffraction (XRD

The XRD analyses on the bulk or pulverulent specimens were carried outat various steps in the synthesis protocol (after calcination, beforesintering or post mortem) and were used to check the nature of thematerial (phase, crystal system) and its evolution according to theprotocol.

Determination of the Substoichiometry by TGA (ThermogravimetricAnalsysis)

The substoichiometry of the material, that is to say the value of δ inthe formula described in this invention, was determined according to thesynthesis protocol employed by measuring the weight loss or increase asa function of the temperature and the oxygen partial pressure. Thepowders have to be dried beforehand so that the change in weight can beascribed only to oxygen exchange with the atmosphere.

The powder or the sintered material reduced to a powder and dried, wasplaced in an alumina crucible in the thermobalance compartment providedfor this purpose. The thermal program and the oxygen partial pressure ofthe medium were controlled in accordance with those of the materialcalcination or sintering protocol. The signal corresponding to thechange in mass recorded as a function of the temperature for a fixedoxygen partial pressure was used to deduce the oxygen substoichiometryof the material.

Analysis of the Oxygen Flux Passing Through the Membrane

Flux tests were carried out with parts in the form of thin disks 30 mmin diameter and between 0.1 and 2 mm in thickness, these being preparedas indicated above.

These membranes were placed within the device as shown in FIG. 4, whichis a schematic sectional representation of the reactor used. Themembranes (1) had a diameter of around 25 mm and a thickness varyingbetween 0.1 and 2 mm. They were positioned individually on the top of analumina tube (2) placed in a thermal chamber (3). The dense alumina tubecontained a controlled atmosphere (4) acting as reducing agent inoperation (inert or reducing gas). The opposite face of the membrane wasswept with an oxidizing atmosphere (5) (air or an atmosphere of variablepO₂). Sealing between the two atmospheres was guaranteed at hightemperature by the presence of an impermeable seal (6) between thealumina tube and the membrane. An oxygen probe or a chromatograph placedin the reducing gas circuit and after the membrane (7) was used tomeasure the oxygen flux through the material.

The oxygen flux was calculated and normalized to the temperature andpressure conditions using the following formula:$J_{O_{2}} = {\frac{C \times D}{S} \times \alpha}$in which:

-   -   J_(O) ₂ is the oxygen flux through the membrane (Sm³/m²/h);    -   C is the O₂ concentration measured at the outlet (ppm);    -   D is the carrier gas flow rate (m³/h);    -   S is the effective area of the membrane (m²); and    -   α is the volume normalization coefficient [for which        T_(normal)=273 K; P_(normal)=10⁵ Pa (1013 mbar)]:        $\alpha = {\frac{P_{measured} \cdot T_{normal}}{P_{normal} \cdot T_{measured}}.}$        Discussion

The influence of the atmospheres used for the heat treatments(calcination, binder removal and sintering) on the ionic conductionproperties of material was mentioned previously. Although theatmospheres of the various heat treatments allow a suitable amount ofoxygen vacancies to be created, the overall stoichiometry of thematerial will not change in operation and will be stable. This isbecause the oxygen leaves the material on the reducing side, but isimmediately replaced with the oxygen from the air on the oxidizing side,so that the overall content of vacancies is unchanged.

It is therefore paramount that the quantity of these vacancies beadjusted before the membrane is used as such.

In the case of the materials according to the present invention, theoxygen substoichiometry is provided by a preparation step, whether thisbe the synthesis (or calcination, step a) and/or sintering (step d) (thelatter including the binder removal cycle of step e)) at hightemperature (>900° C.) in a controlled atmosphere having a lowcontrolled oxygen partial pressure. The thermal chamber may in thisregard be swept with an inert gas (e.g. N₂ or Ar) or a reducing gas(e.g. H₂/N₂ or H₂/He) or it may be in a dynamic vacuum.

Among these possibilities, sweeping the furnace with an inert gas ispreferred.

The blend of precursors may be calcined in air or in an inert gas, andthen sintered in an inert gas (controlled pO₂<0.2). The change in oxygencontent of the lattice may be monitored by XRD (X-ray diffraction) or byTGA (thermogravimetric analysis).

Specifically, the appearance of vacancies in the crystal lattice of thematerial modifies its structure and/or its crystal properties. XRD thenreveals:

either a change in the crystal system (for example from a rhombohedralperovskite for a low vacancy content to a cubic perovskite for a highervacancy content); and

systematically, a change in the lattice parameters, which increase withthe substoichiometry.

FIG. 2 shows the X-ray diffraction diagrams for polycrystallinespecimens and brings out the influence of the oxygen partial pressureduring synthesis on the structure of the material. In this example, thematerial synthesized in air does not have the same crystal system asmaterial synthesized in argon. This in fact shows that all the peaks ofthe material synthesized in argon are narrow whereas some of the peaksof the material synthesized in air are double peaks (they have ashoulder). The material synthesized in argon thus has a cubic symmetrywhereas that synthesized in air has a rhombohedral symmetry.

It is known that the repulsion between cations is greater in asubstoichiometric material, this having the effect of increasing thevolume of the unit cell. As a result, in this diagram all the lines areshifted toward smaller angles.

The loss of oxygen in the material is also manifested by a loss of mass,the amount of which, measured by TGA, allows the final vacancy contentto be estimated.

All the above remarks about the benefit of using synthesis atmosphereshaving a low controlled pO₂ are of course valid only in the case ofmaterials withstanding such atmospheres.

The claimed materials are therefore stable under the temperature andoxygen partial pressure conditions used during the various synthesissteps, that is to say they retain their chemical stability and theiroverall perovskite formula. After the various synthesis steps, it istherefore desirable to check, for example by XRD, that the material hasnot decomposed, either completely or partially.

The synthesis protocol in an atmosphere having a controlled pO₂ alsooffers another advantage, that of greatly reducing the presence ofsecondary phases in the sintered membrane.

This is because the synthesis of a powder from precursors rarely resultsin the formation of a single phase. These secondary phases mayindirectly reduce the performance of the material since their presencemodifies the formulation of the main phase by depleting it of certainelements. Now, as it is difficult to predict in advance what theproportion and the nature of secondary phases will be exactly, theformulation of the final material cannot be guaranteed from anadjustment of the initial amounts of precursors.

The secondary phases included in our materials sintered in air arecompounds of the ABO₃, AB₂O₄, A₂BO₄ type or mixed AA′BO₃, ABB′O₃ orAA′BB′O₃ compounds. Now, for the majority of cases, these phases areunstable at the low oxygen partial pressures, so that the proportion ofsecondary phases is greatly reduced by treatment at a pO₂<2·10⁴ Pa.

FIG. 3 illustrates the influence of the preparation protocol (synthesisand sintering) on the nature of the phases present in the material. Itdemonstrates in particular the benefit of sintering the material at lowoxygen partial pressures in order to favor the presence of asubstoichiometric phase and reduce the presence of inclusions, whichdeplete the material of certain elements on which the conductionproperties depend.

In addition, when the material is sintered in an oxidizing atmosphere,for example in air, the oxygen substoichiometry of material is low,which has negative repercussions on the flux. These negativerepercussions are greater as the sintering in air favors the appearanceof inclusions.

It may be envisaged to subject the material sintered in air to an inertatmosphere before using it as a catalyst, however, the microstructuralchanges that result therefrom cause the membrane to crack.

It is clearly apparent that the search for a mixed conductive perovskitematerial of useful performance cannot be assumed just from itsformulation. The present invention demonstrates the influence of thepreparation protocol on its performance, especially the synthesis step(step a) and/or the sintering step (step d) at low oxygen partialpressures (vacuum, inert or reducing gases).

This change in flux performance (=ionic conductivity of the O²⁻ions+electronic conductivity) is directly due to the presence of oxygenvacancies in the crystal sublattice. The constituent ions of thematerial, for example La³⁺, Sr²⁺, Fe³⁺, Ga³⁺ and O²⁻, are organized in aparticular structure described by a perovskite unit cell. The oxygenanions occupy sites specific to them in this unit cell when one of thesesites is empty—there is therefore a vacancy in the crystal lattice.

When the material is used as a CMR, a difference in partial pressure oneach side of the membrane is the driving force for the diffusion ofoxygen through the crystal lattice, this diffusion being possible onlyat high temperatures. The presence of vacancies within the oxygensublattice increases the diffusion rate of the anions and lowers theactivation energy of (or the temperature for) this diffusion. FIG. 6illustrates the diffusion of oxygen in such a catalytic membranereactor.

It will therefore be understood that the material has to have oxygenvacancies within it in order to be used for a CMR application.

This search for a substoichiometry in the material is firstly achievedby its initial formulation especially by doping the material with anelement likely to create vacancies. Then, secondly, the substoichiometryis obtained by the preparation protocol.

In the example described above, it is strontium that acts as a dopantelement on lanthanum. Sr²⁺ has an ionic radius similar to that of La³⁺,so that it is incorporated into the lattice of the perovskite. However,its charge is different since it possesses an additional electron. Thesubstitution of lanthanum with strontium therefore causes an electronicovercharge, which is immediately compensated for by the crystal so as topreserve its neutrality. According to a first mechanism, thiscompensation is provided by the removal of oxygen, which createspositively charged vacancies so that the positive charges cancel out thenegative charges. The formula is then the following:La_(1-x)Sr_(x)FeO_(3-x/2) or La^((III))_(1-x)Sr^((II))xFe^((III))O^((−II)) _(3-x/2),where x is the degree of substitution of strontium with lanthanum.

The neutrality equation is therefore as follows:3(1−x)+2x+3−2(3x/2)=0

A second mechanism allows the negative charges to be compensated for bythe change in valency of the iron. Iron⁺⁺⁺ captures the excess electronand becomes iron^(IV).

If the change of valency of the iron takes place preferentially in thepresence of a vacancy, the material may be stoichiometric and thus nothave the satisfactory performance. In this case, the formula is:La_(1-x)Sr_(x)FeO₃ or La^((III)) _(1-x)Sr^((II)) _(x)Fe^((III))_(1-x)Fe^((IV)) _(x)O^((−II)) ₃,where x is the degree of substitution of strontium with lanthanum.

The neutrality equation is then given by:3(1x)+2x+3(1−x)+4x−2.(3)=0.

The stoichiometry of the material according to the invention variesbetween the two above extremes, depending on the surrounding oxygenpartial pressure. By controlling the oxygen partial pressure during thevarious steps of preparing the material, and in particular duringcalcination and sintering, it is possible to achieve the optimumsubstoichiometry δ_(opt) and an acceptable performance of conductivity,while preserving the stability of the material. The aim will thereforebe for the material to lie at the maximum shown in the curve in FIG. 7,which illustrates the best flux/stability compromise. The notion ofstability corresponds here to the vacancy content of the material beingpreserved during the operation on which its lifetime will depend.

1-26. (canceled)
 27. A mixed electronic/O²⁻-anion conductive material ofperovskite crystal structure, the electrical neutrality of the crystallattice of which is preserved characterized in that it consistsessentially of a compound of formula (I):A^((a)) _((1-x-u))A′^((a−1)) _(x)A″^((a″)) _(u)B^((b))_((1-s-y-v))B^((b+1)) _(s)B′^((b+β)) _(y)B″^((b″)) _(v)O_(3-δ),  (I) inwhich formula (1): a, a−1, a″, b, b+1, b+β and b″ are integersrepresenting the respective valences of the atoms A, A′, A″, B, B′ andB″; and a, a″, b, b″, β, x, y, s, u, v and δ are such that theelectrical neutrality of the crystal lattice is preserved;a>1;a″, b and b″ are greater than zero;−2≦β≦2;a+b=6;0<s<x;0<x≦0.5;0≦u≦0.5;(x+u)≦0.5;O≦y≦0.9;0≦v≦0.9;0≦(y+v+s)≦0.9;[u(a″−a)+v(b″−b)−x+s+βy+2δ]=0;and δ_(min)<δ<δ_(max) withδ_(min) =[u(a−a″)+v(b−b″)−βy]/2 andδ_(max) =[u(a−a″)+v(b−b″)−βy+x]/2; and in which formula (I): Arepresents an atom chosen from scandium, yttrium or from the families oflanthanides, actinides or alkaline-earth metals; A′, which differs fromA, represents an atom chosen from scandium, yttrium or from the familiesof lanthanides, actinides or alkaline-earth metals; A″, which isdifferent from A and A′, represents an atom chosen from aluminum (Al),gallium (Ga), indium (In) and thallium (Tl); B represents an atom chosenfrom the transition metals that can exist in several possible valences;B′, which differs from B, represents an atom chosen from transitionmetals, aluminum (Al), indium (In), gallium (Ga), germanium (Ge),antimony (Sb), bismuth (Bi), tin (Sn) and lead (Pb); and B″, whichdiffers from B and B′, represents an atom chosen from transition metals,metals of the alkaline-earth family, aluminum (Al), indium (In), gallium(Ga), germanium (Ge), antimony (Sb), bismuth (Bi), tin (Sn) and lead(Pb).
 28. The material as defined in claim 27, for which, in formula(I), δ is equal to an optimum value δ_(opt) that allows it to ensure anoptimum ionic conductivity for sufficient stability under operatingtemperature and pressure conditions as a mixed ionic/electronicconductor.
 29. The material as defined in claim 27, for which, informula (I), a and b are equal to
 3. 30. The material as defined inclaim 27, in which, in formula (I), u is equal to zero.
 31. The materialas defined in claim 27, in which, in formula (I), u is different fromzero.
 32. The material as defined in claim 27, for which, in formula(I), the sum (y+v) is equal to zero.
 33. The material as defined inclaim 27, for which, in formula (I), the sum (y+v) is different fromzero.
 34. The material as defined claim 27, for which, in formula (I), Ais chosen from La, Ce, Y, Gd, Mg, Ca, Sr or Ba.
 35. The material asdefined in claim 34, of formula (Ia):La^((III)) _((1-x-u))A′^((II)) _(x)A″^((a″)) _(u)B^((III))_((1-s-y-v))B^((IV)) _(s)B′^((3+β)) _(y)B″^((b″)) _(v)O_(3-δ)  Ia),corresponding to formula (I) in which a and b are equal to 3 and Arepresents lanthanum.
 36. The material as defined in claim 27, forwhich, in formula (I), A′ is chosen from La, Ce, Y, Gd, Mg, Ca, Sr orBa.
 37. The material as defined in claim 36, of formula (Ib):A^((III)) _((1-x-u))Sr^((II)) _(x)A″^((a″)) _(u)B^((III))_((1-s-y-v))B^((IV)) _(s)B′^((3+β)) _(y)B″^((b″)) _(v)O_(3-δ)  (Ib),corresponding to formula (I) in which a and b are equal to 3 and A′represents strontium.
 38. The material as defined in claim 27, forwhich, in formula (I), B is chosen from Fe, Cr, Mn, Co, Ni and Ti. 39.The material as defined in claim 12, of formula (Ic):A^((III)) _((1-x-u))A′^((II)) _(x)A″^((a″)) _(u)Fe^((III))_((1-s-y-v))Fe^((IV)) _(s)B′^((3+β)) _(y)B″^((b″)) _(v)O_(3-δ)  (IC),corresponding to formula (I) in which b=3 and B represents an iron atom.40. The material as defined in claim 27, for which, in formula (I), B′is chosen from Co, Ni, Ti and Ga.
 41. The material as defined in claim27, for which, in formula (I), B″ is chosen from Ti or Ga.
 42. Thematerial as defined in claim 41, of formula (Id),La^((III)) _((1-x))Sr^((II)) _(x)Fe^((III)) _((1-s-v))Fe^((IV))_(s)B″^((b″)) _(v)O_(3-δ)  (Id), corresponding to formula (I) in whicha=b=3, u=0, y=0, B represents an iron atom, A is a lanthanum atom and A′is a strontium atom.
 43. The material as defined in claim 27, for which,in formula (I), A″ is chosen from Ba, Al and Ga.
 44. The material asdefined in claim 27, for which formula (I) is either:La^((III)) _((1-x-u))Sr^((II)) _(x)Al^((III)) _(u)Fe^((III))_((1-s-v))Fe^((IV)) _(s)Ti_(v)O_(3-δ),La^((III)) _((1-x-u))Sr^((II)) _(x)Al^((III)) _(u)Fe^((III))_((1-s-v))Fe^((IV)) _(s)Ga_(v)O_(3-δ),La^((III)) _((1-x))Sr^((II)) _(x)Fe^((III)) _((1-s-v))Fe^((IV))_(s)Ti_(v)O_(3-δ),La^((III)) _((1-x))Sr^((II)) _(x)Fe^((III)) _((1-s-v))Fe^((IV))_(s)Ga_(v)O_(3-δ), orLa^((III)) _((1-x))Sr^((II)) _(x)Fe^((III)) _((1-s))Fe^((IV))_(s)O_(3-δ).
 45. The material of formula (Id) as defined in claim 42, inwhich x is equal to 0.4, B″ represents a trivalent gallium atom, v isequal to 0.1 and δ=0.2−(s/2) and δ is preferably equal toδ_(opt)=0.180±0.018.
 46. A method of preparing a mixed electronic/O²⁻anion conductive material of perovskite crystal structure, theelectrical neutrality of the crystal lattice of which is preserved,represented by the crude formula (I′):A_((1-x-u))A′_(x)A″_(u)B_((1-y-v))B′_(y)B″_(v)O_(3-δ),  (I′) in whichformula (I′): x, y, u, v and δ are such that the electrical neutralityof the crystal lattice is preserved;0<x≦0.5;0≦u≦0.5;(x+u)≦0.5;0≦y≦0.9;0≦v≦0.9;0≦(y+v)≦0.9; and0<δ and in which formula (I′): A represents an atom chosen fromscandium, yttrium or from the families of lanthanides, actinides oralkaline-earth metals; A′, which differs from A, represents an atomchosen from scandium, yttrium or from the families of lanthanides,actinides or alkaline-earth metals; A″, which is different from A andA′, represents an atom chosen from aluminum (Al), gallium 9Ga), indium(In) and thallium (Tl); B represents an atom chosen from the transitionmetals that can exist in several possible valences; B′, which differsfrom B, represents an atom chosen from transition metals, aluminum (Al),indium (In), gallium (Ga), germanium (Ge), antimony (Sb), bismuth (Bi),tin (Sn) and lead (Pb); and B″, which differs from B and B′, representsan atom chosen from transition metals, metals of the alkaline-earthfamily, aluminum (Al), indium (In), gallium (Ga), germanium (Ge),antimony (Sb), bismuth (Bi), tin (Sn) and lead (Pb); characterized inthat it comprises the following successive steps: a step (a) ofsynthesizing a powder having an essentially perovskite crystal phasefrom a blend of compounds consisting of at least one carbonate and/or ofan oxide and/or of a sulfate and/or of a nitrate and/or of a salt ofeach of the elements A, A′ and B and, if necessary, of a carbonateand/or of an oxide of A″, B′ and/or B″; a step (b) of forming the powderblend obtained from step (a); a step (c) of removing the binder from theformed material obtained from step (b); and a step (d) of sintering thematerial obtained from step (c); and characterized in that at least oneof steps (a), (c) and (d) is carried out while controlling the oxygenpartial pressure (pO₂) of the gaseous atmosphere surrounding thereaction mixture.
 47. The method as defined in claim 46, characterizedin that step (c) is carried out while controlling the oxygen partialpressure (pO₂) of the gaseous atmosphere surrounding the material fromwhich the binder is to be removed.
 48. The method as defined in claim46, in which step (d) is carried out in a gaseous atmosphere having anoxygen partial pressure not exceeding 0.1 Pa.
 49. The method as definedin claim 48, in which step (a) is carried out in air.
 50. A mixedelectronic/O²⁻ anion conductive material of perovskite crystalstructure, the electrical neutrality of the crystal lattice of which ispreserved, represented by the crude formula (I′):A_((1-x-u))A′_(x)A″_(u)B_((1-y-v))B′_(y)B″_(v)O_(3-δ),  (I′) in whichformula (I′): x, y, u, v and δ are such that the electrical neutralityof the crystal lattice is preserved;0<x≦0.5;0≦u≦0.5;(x+u)≦0.5;0≦y≦0.9;0≦v≦0.9;0≦(y+v)≦0.9; and0<δ and in which formula (I′): A represents an atom chosen fromscandium, yttrium or from the families of lanthanides, actinides oralkaline-earth metals; A′, which differs from A, represents an atomchosen from scandium, yttrium or from the families of lanthanides,actinides or alkaline-earth metals; A″, which is different from A andA′, represents an atom chosen from aluminum (Al), gallium (Ga), indium(In) and thallium (Tl); B represents an atom chosen from the transitionmetals that can exist in several possible valences; B′, which differsfrom B, represents an atom chosen from transition metals, aluminum (Al),indium (In), gallium (Ga), germanium (Ge), antimony (Sb), bismuth (Bi),tin (Sn) and lead (Pb); and B″, which differs from B and B′, representsan atom chosen from transition metals, metals of the alkaline-earthfamily, aluminum (Al), indium (In), gallium (Ga), germanium (Ge),antimony (Sb), bismuth (Bi), tin (Sn) and lead (Pb); and in which δdepends on the oxygen partial pressure in the gaseous atmospheres inwhich steps (a), (d) and optionally (c) of the method as defined in oneof claims 20 to 23 take place.
 51. Use of the material as defined inclaim 27 as mixed conductive material of a catalytic membrane reactordesigned to be used to synthesize syngas by the oxidation of methane ornatural gas.
 52. Use of the material as defined in claim 27 as mixedconductive material of a ceramic membrane designed to be used toseparate oxygen from air.